1. Field of the Invention
This invention relates generally to an electronic circuit for sampling and holding a digital signal and, more particularly, to an electronic circuit for sampling a digital signal and for replicating the sampled digital signal during a select hold period.
2. Description of the Prior Art
Apparatus for sampling an analog electronic signal and for replicating the sampled analog signal during a selected hold period are well known in the art. Such apparatus may be utilized in photographic exposure control systems in a manner as is more fully described in U.S. Pat. No. 4,192,587, entitled "Proportional Fill Flash", by A. G. LaRocque et al., issued Mar. 11, 1980, in common assignment herewith, wherein the analog output signal from a scene light detecting and integrating circuit is continuously sampled during the exposure interval. The photographic apparatus utilizes a strobe as the primary source of illumination under conditions where the ambient scene light is low or as a source of fill-in illumination under conditions where the ambient scene light is high. Upon the firing of the strobe, it is preferred that the artificial scene light from the strobe not be detected and integrated by the scene light detecting and integrating circuit. Since it is physically impossible to block the artificial scene light from the strobe from reaching the scene light detecting photocell while simultaneously admitting the ambient scene light to the photocell, the analog output signal from the scene light detecting and integrating circuit must be blocked for the duration of the pulse of artificial light and a replicated signal substituted therefor representative of the analog signal at the instant immediately prior to the strobe ignition. Toward this end, there is provided a sample and hold circuit wherein the output analog signal from the scene light detecting and integrating means is continuously sampled until the firing of the strobe at which instant the sampled signal is held and substituted in place of the actual signal from the scene light detecting and integrating circuit for the duration of the strobe ignition time.
In some photographic exposure control systems, however, it has been found preferable to utilize a scene light detecting and integrating circuit which provides a digital type output signal instead of the analog output signal previously discussed. Such digital scene light detecting and integrating circuits may be of a type disclosed in U.S. Pat. No. 3,818,204, entitled "Voltage Integrating Apparatus", by R. Hadida, issued June 18, 1974. Sampling and holding a digital signal, however, is a substantially more complicated procedure than that required to sample and hold the aforementioned analog type signal and involves the use of digital type counters and registers which as a result of their finite counting capacity may rollover prematurely to provide an erroneous count under conditions where the period of the sampled digital signal becomes extremely long.
Therefore, it is a primary object of this invention to provide an electronic circuit for sampling a digital signal and for replicating the sampled digital signal during a selected hold period without incurring the risk of counters embodied therein rolling over.
It is a further object of this invention to provide a photographic exposure control system wherein the scene light is detected and integrated to provide a digital signal which is subsequently sampled with the sampled signal being substituted for the actual digital signal as a replication thereof during the flash fire time so as to effectively blank the detection and integration of the artificial light provided by the strobe.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a mechanism and system possessing a construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.